1. Field of the Invention
The invention relates to a metal halide lamp of the short arc type which is used as a light source of an overhead projector (OHP), a direct projector, and the like.
2. Description of Related Art
Conventionally, an overhead projector is known as an information presentation tool. However, recently, a liquid crystal projector which projects a liquid crystal display, and a direct projector have become common; only by simply placing an article to be projected on a sample carrier can the direct projector project its color and shape as it is. Conventionally, halide lamps are used for light sources of these projection devices. However, recently, instead of halogen lamps, metal halide lamps of the short arc type have been used because in halogen lamps there are the disadvantages of more frequent bulb changing than in discharge lamps, low radiant efficiency, image brightness on the projection screen which is inadequate due to its yellowish, warm-color emission in a bright room, and the like.
In a metal halide lamp of the short arc type, a pair of electrodes disposed opposite one another within an arc tube at a distance of a few millimeters, encapsulated together with mercury as a buffer metal, a starting rare gas, and metal halides as emission metals. The metal halides melt during luminous operation and are present on the wall of the arc tube as a liquid, while they partially vaporize as a gas, and dissociate into metal atoms and halogen in the high temperature range of the arc center, the metal elements being excited by the arc and emitting spectra which are typical of the metals.
In a metal halide lamp, the metal halides vaporize in this way. Therefore, a vapor pressure is obtained to a sufficient degree at a temperature which is lower than in the metal elements. Furthermore, better radiant efficiency than in a high pressure mercury lamp, and in addition, good color reproduction can be obtained by means of suitable selection of the metals to be encapsulated,.
In the above described projection device, the liquid crystal projector in which a metal halide lamp is used as the light source has a red color filter, a green color filter and a blue color filter. By combining the light from the lamp with these color filters, good color reproducibility can be achieved on the screen surface. In an OHP or direct projector, however, the light does not pass from the metal halide lamp through the filter, but is projected directly. Therefore, here, there is a need for more exact color reproducibility than in a liquid crystal projector.
If, for example, a metal halide lamp which is often used for a light source of a liquid crystal projector and in which halides of dysprosium, neodymium and cesium are encapsulated as the emission halides is used for the light source of an OHP or direct projector, the proportion of green color to which an individual reacts sensitively increases in the image projected onto the screen, and the component of blue decreases, resulting in the disadvantage that sufficient color reproducibility cannot be obtained.